1. Field of the Invention
The present invention relates in general to purifying aromatic dicarboxylic acids and more specifically to improving catalyst selectivity during aromatic dicarboxylic acid purification such that the aromatic dicarboxylic acid product is not hydrogenated.
2. Description of the Related Art
Aromatic dicarboxylic acids are used to produce a variety of polyester products. Aromatic dicarboxylic acids are generally synthesized by the catalytic oxidation of the corresponding aromatic dialkyl compound. For example, terephthalic acid (TPA) and isophthalic acid (IPA) are produced by the liquid phase oxidation of p-xylene and m-xylene, respectively, by the following reactions. ##STR1##
In the above reactions, Co/Mn/Br may be used as the catalyst. The above reactions work well. However, in addition to producing the aromatic dicarboxylic acids, a number of impurities are also produced. The following are the impurities produced in the catalytic oxidation of isophthalic acid: ##STR2##
Carboxybenzaldehyde (CBA) and toluic acid result from the incomplete oxidation of the aromatic dimethyl compound. In the oxidation of m-xylene to produce IPA, 3-CBA is produced. In the oxidation of p-xylene to produce TPA, 4-CBA is produced. Likewise, m-toluic acid is an impurity in IPA production and P-toluic acid is an impurity in TPA production. Because neither the CBA nor the toluic acid have two carboxylic acid groups, both would terminate the chain of a polyester produced from a crude dicarboxylic acid. Thus, both CBA and toluic acid are undesirable. However, toluic acid is only produced in small quantities and is water soluble, thus removable in a crystallization step.
In addition to the CBA and toluic acid impurities, compounds generally known as "fluorenones" are produced. The fluorenone shown above is only one of several isomers. Fluorenones have two carboxylic acid groups, and are therefore not chain terminating. However, fluorenones are yellow. Thus, if fluorenones are present, the polyester produced from the aromatic dicarboxylic acid will appear dingy.
In view of the foregoing, it is necessary to purify crude aromatic dicarboxylic acids. The dicarboxylic acids are purified by catalytic hydrogenation of the impurities in the following reactions. ##STR3##
As can be seen above, fluorenones are converted to "fluorenes", and CBAs are converted to toluic acid and hydroxymethylbenzoic acid (HMBA). Fluorenes are bifunctional, thus not polymer chain terminating, and are white. The purification is generally carried out by dissolving the oxidation products in water at an elevated temperature and pressure, followed by contacting the resulting solution with a bed of hydrogenation catalysts in the presence of a partial pressure of hydrogen. The product mixture is allowed to cool which causes the purified product to crystallize. The toluic acid and HMBA remain in solution. The hydrogenation catalyst is commonly palladium on a carbon (charcoal) support, which catalyst contains 0.5 weight percent palladium.
One of the disadvantages of the purification process is the tendency to hydrogenate the aromatic dicarboxylic acid to produce undesired by-products. In the production of CBA, the following undesired hydrogenation reactions may occur. ##STR4##
As can be seen, IPA may be hydrogenated to cyclohexane dicarboxylic acid (CHDA) and cyclohexane carboxylic acid (CHCA). Only a small amount of CHCA is produced. IPA may also be hydrogenated to m-toluic acid, but little m-toluic acid is produced on this route. Note that benzoic and toluic acids may also be produced from CBA. The by-products are water soluble, and thus are not overly difficult to remove. However, the by-products represent a yield loss of the desired bromate dicarboxylic acid.
Most past efforts aimed at decreasing the amount of excess CHDA and CHCA produced during the purification centered around the addition of rhodium to the palladium/carbon catalyst, which is typically used in the purification process. For example, U.S. Pat. No. 4,394,299 teaches the use of a by-metallic Pd/Rh-on-carbon catalyst for the purification of terephthalic acid to decrease the amount of 4-CBA and minimize the amount of CHDA byproduct. U.S. Pat. Nos. 4,629,715 and 4,892,972 teach the use of layered catalyst beds consisting of a layer of Rh/C catalyst before or after the bulk of Pd/C catalysts. However, rhodium is very expensive.